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vinyl: clean-up write iterator if vy_task_write_run() fails

In vy_task_write_run if write_iterator->start() fails, there's no call
of corresponding stop() method. vy_task_write_run() is executed in
auxiliary thread (dump or compaction). Meanwhile, creating/destroying
tuples in these threads does not change reference counter of corresponding
tuple formats (see vy_tuple_delete() and vy_stmt_alloc()).
Without cleaning up write iterator right in vy_task_write_run() after
fail, this procedure takes place in vy_task_compaction_abort() and
vy_task_dump_abort(). These *_abort() functions in turn are executed in
the main thread. Taking this into consideration, tuple may be allocated
in aux. thread and deleted in the main thread. As a result, format
reference counter decreases, whereas it shouldn't change (otherwise
tuple format will be destroyed before all tuples of this format are gone).

Real example of the bug described above can be achieved in the following
way:
1. run compaction process;
2. add one or more slice sources in vy_write_iterator_start():
corresponding slice_stream structures obtain newly created tuples
in vy_slice_stream_next();
3. the next call of vy_write_iterator_add_src() fails due to OOM,
invalid run file or whatever;
4. since there's no clean-up of tuples in slice streams, they are
destroyed in vy_task_compaction_abort() in the main thread exit;
5. now format reference counter is less than it was before compaction.

Closes #4864

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